1. Field of the Invention
The present invention relates to a method and an apparatus for reducing optical noise generated by a semiconductor laser in an optical information processing system such as an optical disc player which utilizes a semiconductor laser as a light source.
2. Description of the Prior Art
A single longitudinal mode semiconductor laser which oscillates at one wavelength has a very small astigmatism and hence can highly converge a light spot emitted from one facet of a device at an external record medium. For example, in an optical disc player, particularly a digital audio disc player, it is prescribed that a light spot diameter .lambda./NA on a disc surface is no larger than 1.75 .mu.m. When the single longitudinal mode semiconductor laser is used, the light spot diamter .lambda./NA can be 0.79/0.47 =1.68 .mu.m. By converging the light spot in this manner, a recorded signal on the disc can be more exactly read.
However, in such a single longitudinal mode semiconductor laser, it has been known that when a light emitted from the one facet of the device is reflected by the external record medium and fed back to the emitting one facet, a light output fluctuates even if the laser device is driven by a constant current. An optical noise appears in reproducing a signal so that a signal cannot be detected with a high fidelity.
Thus, in an optical information apparatus such as a fiber optic communication apparatus or an optical disc player which uses an semiconductor laser, an optical isolator is used to minimize a light fed back to the semiconductor laser device in order to prevent the semiconductor laser noise due to the feedback light. However, it is practically difficult to completely block the feedback light and a certain amount of feedback light to the semiconductor laser is inevitably included due to a precision error of the optical parts and a variation among the parts. The magnitude of the optical noise significantly changes when a small change occurs in the amount of feedback light. When the amount of feedback light reaches approximately 0.1% of the amount of emitted light, the optical noise abruptly increases. Accordingly, in the past, it has been required to increase the precision of the optical parts, reduce the variation among the parts and increase the precision in assembly of the parts. Therefore, the prior art apparatus is not suitable for mass production.
FIG. 1a shows a basic construction of an optical system of an optical disc player pickup. A laser beam emitted from an emitting point 8 of a semiconductor laser 1 is collimated by a coupling lens 2. The collimated light beam passes through a polarization beamsplitter 3 and a quarter wave plate 4 and then is converged by a objective lens 5 into a light spot 9 on a reflection plane 6 of the disc. The light reflected by the reflection plane again passes through the objective lens 5 and the quarter wave plate 4; is deflected normally by the polarization beamsplitter 3; and then reaches a light detector 7 which converts the light to an electric signal.
In the prior art, when the semiconductor laser of this type is used, the combination of the quarter wave plate 4 and the polarization beamsplitter 3 is principally designed to prevent the feedback of the reflected light from the disc to the semiconductor laser. As shown in FIG. 1b, the linearly polarized light beam emitted from the semiconductor laser is applied to the quarter wave plate 4a with a polarization plane thereof being at 45 degrees to a crystal axis of the quarter wave plate so that the laser beam passed through the quarter wave plate is circularly polarized. When this laser beam is reflected by the reflection plane and again passes through the quarter wave plate, it is linearly polarized with the polarization plane thereof being rotated by 90 degrees with respect to that of the original beam. The light beam having the polarization of 90 degrees does not transmit through the polarization beamsplitter but it is reflected thereby so that the light beam is not fed back to the semiconductor laser. However, it is inevitable that the laser beam is fed back to the emitting point 8 because of a precision error of the optical parts, a precision error in the assembly of the optical parts and an optical anisotropy (birefraction). For example, assuming that a ratio of light distributed to the light detector 7 to that to the semiconductor laser 1 is 200 when the reflected light from the reflection plane 6 of the disc in FIG. 1a is best directed to the light detector 7 by the polarization beamsplitter 3 and a coupling efficiency between the semiconductor laser 1 and the coupling lens 2. That is, a ratio of a total amount of light emitted forwardly of the semiconductor laser to the amount of light passed through the coupling lens 2 is 20% and a reflection factor of the reflection plane is 90%, approximately 0.09-0.1% of the light emitted forwardly of the semiconductor laser is fed back to the emitting point 8. This feedback light causes the increase of the optical noise. It is, therefore, necessary to precisely control the tolerance of the optical parts and the variation among the parts to minimize the amount of feedback light.